Procedure for the production of high-purity melamine with high yields

ABSTRACT

A process for the production of high-purity melamine by pyrolysis of urea at high pressure, wherein the liquid phase output from the pyrolysis reactor is sent to a downstream post-reactor, and wherein the anhydrous gaseous phase from the pyrolysis reactor and from the post-reactor are subjected to washing with molten urea for the recovery of the melamine. The purified liquid exiting the post-reactor is treated in a quenching column, in order to eliminate polycondensates, while the quenching column output is cooled. A high purity melamine is then separated by crystallization from a mother liquor, the greater part of which is recycled to the quenching column, thereby enabling a costless recovery of ammonia and melamine. The remaining mother liquor is treated for the separation of oxidized products of pyrolysis and is then sent back to the quenching column, thereby realizing the complete recovery of the melamine.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved procedure for theproduction of high-purity melamine according to a process based on thepyrolysis of urea under high pressure.

In greater detail, the present invention refers to the process thatprovides for the collection and purification in aqueous solution ofmelamine produced in a reactor and its separation by crystallization.

2. Description of Related Art

The reaction transforming urea into melamine follows the stoichiometryshown in the following equation:

$\begin{matrix}\left. {6{NH}_{2}{CONH}_{2}}\rightarrow{{({CN})_{3}\left( {NH}_{2} \right)_{3}} + {6{NH}_{3}} + {3{CO}_{2}}} \right. \\{{urea}\mspace{146mu}{melamine}\mspace{50mu}{ammonia}\mspace{20mu}{carbon}\mspace{14mu}{dioxide}}\end{matrix}$according to which approximately 1.86 kilograms of a gaseous mixture ofNH₃ and CO₂ (collectively called off-gas) are formed for every kilogramof melamine produced.

The most widely used process based on the pyrolysis of urea under highpressure is described in U.S. Pat. No. 3,161,638 to Allied. The basis ofthis process is that all effluents from the melamine synthesis reactorare cooled and collected in aqueous ammonia medium. The presence ofammonia as alkaline medium prevents the precipitation of theintermediate oxidation products of the pyrolysis reaction, calledoxyaminotriazines (OAT) and enables the transformation into melamine ofthe de-ammoniating condensation by-products of the same(polycondensates), thus assuring a high degree of purity of the product.

According to this process, a stream of off-gas is also producedcontaining water vapor derived from the aforementioned treatment of theentire effluent of the reactor with an aqueous medium. This gaseousphase is normally returned to the urea synthesis plant in order torecover the NH₃ and the CO₂ contained in it. However, the presence ofwater vapor in the off-gas stream can constitute a problem for the ureaplant.

Furthermore, the residual aqueous solution (mother liquor) separatedfrom the crystallized melamine according to this process cannot bedirectly recycled and reused to dissolve the melamine coming out of thereactor, because the concentration of OAT would increase continuouslyand, once the aqueous medium becomes saturated, OAT would precipitatetogether with the melamine crystals contaminating the product. For thisreason, the mother liquor must be suitably treated before beingrecycled, in order to separate OAT and maintain their concentration inthe mother liquor at a constant level below the solubility limit.

The aforesaid treatment not only renders the aqueous cycle of collectionand purification of the melamine more complex, but also adds a source ofcosts, both in terms of investment and of energy consumption.

A schematic illustration of an embodiment of the process according tothe above-mentioned U.S. Pat. No. 3,161,638 is shown in FIG. 1,representing the state of the art for the present invention, in order todemonstrate the advantages of the improvements to the aforesaid processbrought about by the present invention.

According to the outline in FIG. 1, the urea is fed as a liquid at atemperature of 135–140° C. to a pyrolysis vat Reactor that workscontinuously and in which a suitable heating system supplies thenecessary calories to the reacting system, maintaining it at atemperature of 360–420° C. The reaction pressure is maintained at avalue above 7 MPa. The reactor is single-stage, and the reacting mass ismaintained in strong circulation by the gases that are formed during thepyrolysis of the urea. The reacted mass (liquid and gas) is continuouslydischarged into an apparatus (Quench) where its temperature is loweredto approximately 160° C. in the presence of a water solution.

Under these conditions, all the melamine, the non-reacted urea and thevarious impurities, pass into solution and are sent downstream to beprocessed, while a gaseous phase, consisting substantially of NH₃ andCO₂, is separated and recycled to the urea synthesis plant, togetherwith the amount of water vapor corresponding to the thermodynamicequilibrium in the Quench condition.

The aqueous solution from the Quench also contains a certain amount ofdissolved ammonia and CO₂ that is eliminated in the following CO₂Stripper. The elimination of the CO₂ is necessary in order to get a highdegree of purity of melamine in the down-stream treatment.

The aqueous stream from the bottom of the CO₂ Stripper, containing aresidual amount of CO₂ of 0.3–0.5% by weight, contains melamine at aconcentration of 6–12% by weight, together with OAT and thepolycondensates. The polycondensates, given their low solubility, mustbe eliminated before sending said aqueous stream to the Crystallizer forthe recovery of the melamine.

In order to eliminate the polycondensates, the solution is heated toapproximately 170° C. in the presence of ammonia in a suitable column,called Hydrolizer, in which ammonia is added to the warm solution untilit reaches the level of 12–15% by weight. During the stay in theHydrolizer under these conditions, the polycondensates are almosttotally transformed into melamine and, to a lesser extent, into OAT.

The purified ammoniacal solution from the Hydrolizer is fed to theCrystallizer where the temperature is lowered to 40–50° C., thusallowing the crystallization of the greater part of the melamine. Thepresence of ammonia in the Crystallizer serves to maintain OAT insolution and thus to separate a product characterized by a high degreeof purity (+99.9% weight). In the following operation of Liquid/SolidSeparation, the crystallized melamine is separated from an aqueousstream containing the OAT formed in reaction and in the various piecesof equipment of the aqueous circuit due to the hydrolysis of melamine.

This aqueous stream (called mother liquor), in which the residualmelamine is present at a concentration of 0.8–1% by weight, cannot berecycled directly to the Quench because, otherwise, the OAT contentwould continue to increase and, once the saturation concentration isreached, they would precipitate in the Crystallizer contaminating theproduct. On the other hand, the mother liquor cannot be discharged intothe ambient because of the presence of large amounts of ammonia andother organic materials. Other than that, dumping the mother liquorwould correspond to a heavy economic loss because of its melamine andammonia content.

Therefore, the process according to U.S. Pat. No. 3,161,368 provides forthe treatment of the mother liquor in De-Ammoniating Column. Here theammonia is completely recovered and an ammonia-free solution isproduced; this solution contains almost exclusively melamine and OAT.Cooling this solution to room temperature causes the Precipitation andSeparation of OAT, which are thus eliminated from the aqueous cycle,allowing the re-circulation of the purified mother liquor and therecovery of the melamine contained in the mother liquor.

The process illustrated here above is currently in use industrially innumerous plants, but requires a certain consumption of steam, becausealmost the entire amount of the OAT-containing mother liquor must beheated. Furthermore, the presence of water in the stream of ammonia andcarbon dioxide (off-gas) that are returned to the urea synthesis plantrequires some adjustments in the operative conditions of the plant.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is a process for the production ofmelamine, which is greatly improved compared to the state of the artbecause: a) a substantially total recovery at low cost of the melaminedissolved in the mother liquor is enabled; b) a gaseous streamcontaining CO₂ and NH₃ completely free of water is separated andreturned to the urea synthesis plant; c) a substantial increase in thetotal yield of the plant in terms of urea consumption is provided; d)the energy consumption of the entire process is considerably reduced; e)the number of processing steps is reduced, increasing the stream factorand the investment cost of the plant.

The object of the invention is achieved by a process that introducessimple but substantial modifications to the state of the art.

The main variations consist of the separation of off-gases from theproduct of pyrolysis of the urea before the treatment of melamine in theaqueous medium, and the introduction of a Post reactor, downstream ofthe pyrolysis Reactor, fed with the liquid phase containing all themelamine produced. The employment of a Post-reactor allows ureaconversion to reach practically 100% and the amount of OAT produced inthe reaction to be drastically reduced. The reduction of OAT allows, inthe down stream, most of the mother liquor to be recycled directly tothe Quenching Column without any treatment, permitting the directrecovery, without loss, of the corresponding amount of ammonia andmelamine contained. Accordingly, the mother liquor, treated in theDe-Ammoniating Column and in the OAT Precipitation and SeparationSection, can be drastically reduced in comparison to the actual state ofthe art, with consequent reduction of investment and energy consumption.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The process of melamine synthesis according to the invention and theadvantages that derive from it can best be understood from the followingdescription of an embodiment of the present invention, with the aid ofthe attached FIG. 2. The description and the related outline of theprocess should not be considered as limiting the scope of the invention.

FIG. 1 is a schematic illustration of a process for the production ofmelamine according to the prior art.

FIG. 2 is a schematic illustration of the main equipment and flow linesof the products and reagents of the process according to one embodimentof the invention.

DETAILED DESCRIPTION OF THE INVENTION

The Reactor of FIG. 2 works in the same conditions as in FIG. 1, withthe difference that there are two separate outputs from the Reactor: theoff-gases output and the raw liquid melamine output. Neither of theseoutputs contains water.

The gaseous anhydrous stream of NH₃ and CO₂ containing vaporizedmelamine, in a proportion according to its vapor pressure, is fed to anoff-gas Washing Section, operating at the same pressure as the pyrolysisreactor. In the off-gas Washing Section, the melamine is removed bydirect contact with molten urea fed at a temperature of 135–140° C. Bythis operation, in which the considerable heat content of the off-gasesis recovered as steam, the melamine is completely recovered by themolten urea and the resulting liquid mixture constitutes the input tothe pyrolysis Reactor. The off-gases thus purified, leaving the WashingSection at a temperature of 170–200° C., are returned to the urea plantfor the total recovery of NH₃ and CO₂. In a variant of this process, thegaseous fraction formed in the pyrolysis reaction can be separated in asuitable vessel placed downstream of the Reactor or in the samePost-reactor that, in such a case, also acts as a separator.

The molten urea containing the melamine recovered from the WashingSection feeds the Reactor by gravity or by means of a suitable pump.

The liquid stream of raw melamine leaving the Reactor is sent to thePost-reactor, which works under the same conditions of temperature andpressure as the Reactor, where it comes into intimate contact withsuperheated anhydrous gaseous ammonia added in amounts equal to from1:10 to 1:1, typically 1:3 by weight of the raw liquid melamine.

The superheated ammonia passing through the liquid mass of the rawmelamine extracts the dissolved CO₂ and enables the completetransformation of OAT into melamine.

The residence time of the melamine stream in the Post-reactor can varyfrom 0 to 2 hours, preferably from 15 to 45 minutes, obtaining areduction of the OAT content to a value below 6,000 ppm.

This additional residence time of the liquid raw melamine stream in thePost-reactor also provides for the completion of the conversion of theurea. Moreover, with the elimination of the CO₂, the partial pressure ofammonia in the Post-reactor increases with consequent reduction of thepolycondensates concentration.

The purified melamine exits from the Post-reactor containing reducedamounts of OAT and polycondensates and practically no residual urea.

The superheated ammonia blown into the Post-reactor remainssubstantially in the vapor phase and, exiting from the Post-reactorseparately from the purified liquid melamine, joins the output gas fromthe Reactor before the off-gases Washing Section, where the recovery ofthe melamine present in the vapor phase occurs.

The purified liquid melamine stream is fed to the Quenching Columnwhich, due to the absence of a gaseous phase, operates completely fullof liquid. The purified liquid melamine enters at the bottom of theQuenching Column, which is kept strongly agitated by mechanical means,and is brought into intimate contact at a temperature of 160–170° C.with a water and ammonia solution coming from the purification circuit.The solution thus formed, in which the NH₃ content is maintained at avalue above 10% by weight, proceeds upwards for sufficient contact timeto transform the residual polycondensates coming from the Post-reactorinto melamine. Since the concentration of CO₂ in the Quenching Column isvery low (less than 0.1% by weight), effective hydrolysis ofpolycondensates is obtained in this column with very short contact times(under 30 minutes).

The aqueous solution of melamine from the head of the Quenching Columnis fed directly to the Crystallizer where the temperature is lowered to40–50° C., causing precipitation of crystals of very high-puritymelamine, which are separated from the mother liquor in the next stage,the Liquid/Solid Separator.

The mother liquor in output from the Liquid/Solid Separator contains areduced amount of OAT, far below the saturation value, since the OAThave been drastically reduced in the Post reactor. Therefore, thegreatest part of the mother liquor can be recycled to the QuenchingColumn, without any treatment, with no risk of OAT reaching thesaturation value in the Crystallizer with consequent precipitation.

According to the present embodiment, in order to stabilize theconcentration of OAT in the circulating aqueous solution at a valueprudentially far away from the value corresponding to the saturation inthe Crystallizer condition, it is sufficient to send only a small partof the mother liquor to the De-Ammoniating Column and to the OATPrecipitation and Separation system. The portion of mother liquorundergoing treatment is in fact less than 20% of the output from theLiquid/Solid Separator. The less the amount of OAT output from thePost-reactor, the smaller this quantity will be. In other words, thehigher the efficiency of the Post-reactor in reducing OAT, the smallerthe portion of mother liquor which must be sent to the treatment and thegreater the economic saving obtained in terms of investment cost andenergy consumption.

The portion of mother liquor not directly recycled to the QuenchingColumn is subject to the same treatment as in the cycle illustrated inFIG. 1 and, after the separation and the recovery of the ammonia in theDe-Ammoniating Column and the elimination of OAT in the OATPrecipitation and Separation section, that portion is also recycled tothe Quenching Column, thereby enabling the total recovery of themelamine and ammonia. The concentration of CO₂, present in minimalamount in the purified output from the Post-reactor, is maintainedconstant in the circulating aqueous solution by continuously extractinga stream rich of CO₂ from a suitable point of the De-Ammoniating Column.

The present embodiment provides the following, considerable advantages:

1. The production of off-gases without water (anhydrous off-gases) andat a higher pressure facilitates their recovery in the urea plant towhich they are returned. The economic value of anhydrous off-gases ishigher than that of wet, lower pressure, off-gases produced by theprocess in the prior art.

2. The simplification of the aqueous purification circuit and thedrastic reduction in the dimensions of the De-Ammoniating Column and theOAT Precipitation and Separation Section involve a net reduction of theinvestment cost, which exceeds the added investment for the Post-reactorvessel and the off-gas Washing Section. The system based on the processin the present embodiment provides for an investment reduction in excessof 15% compared with a plant in the prior art.

3. The total conversion of urea and the almost complete transformationof OAT into melamine in the Post-reactor involve an increase in overallyield of the process, corresponding to a reduction of at least 8% inconsumption of the urea compared to the existing technology.

4. A further increase in overall yield of the process is due to thereduction of hydrolysis of the melamine into OAT in the aqueous cycle,because of the smaller numbers and volume of the pieces of equipment, inwhich the melamine remains in contact with aqueous solutions at a hightemperature. The reduction of number and volume of the pieces ofequipment in the aqueous cycle is consequent to the simplification ofthe same cycle and the drastic reduction in capacity of theDe-Ammoniating Column.

5. The direct recycling of the greatest part of mother liquor from theLiquid/Solid Separator to the Quenching Column (with the consequentreduction of the fraction to be treated in the De-ammoniating column),provides for a reduction in energy consumption of more than 40% comparedto the current process.

EXAMPLE

In a high-purity melamine production plant built in accordance with oneembodiment of the invention and comprising all the stages included inthe process of FIG. 2, 950 kg/hr of molten urea at a temperature of 135°C. are introduced into the Washing Section of the anhydrous off-gasescoming from the reaction section.

The Washing operation is conducted at a pressure of 8 MPa and at atemperature of 185° C.

From the Washing Section are obtained 746 kg/h of anhydrous off-gases,free of melamine, that are sent to the adjacent urea synthesis plant,and a liquid mixture containing urea and the recovered melamine that isfed by gravity to the Reactor.

In the Reactor, the temperature is maintained at 380° C. and thepressure to 8 MPa for a residence time (calculated according to theentering molten urea stream) of approximately 50 minutes.

From the Reactor exit a raw melamine liquid stream containing 91% byweight of melamine and, separately, a gaseous mixture of CO₂ and NH₃saturated with melamine vapors, which is sent to the off-gas WashingSection for the recovery of the melamine. The liquid stream is fed tothe Post-reactor where, under the same conditions as in the Reactor, itis treated with a gaseous stream of 100 Kg/h of superheated anhydrousammonia that almost totally eliminates the dissolved CO₂. The residencetime of the liquid melamine in this piece of equipment is 45 minutes.The ammonia and gaseous CO₂ output from the Post-reactor joins theoff-gases output from the Reactor and are fed together to the off-gasWashing equipment.

Molten melamine flows from the Post-reactor containing approximately6000 ppm by weight of OAT and less than 1% by weight of polycondensates.

This purified melamine stream is fed to the Quenching Column, where itpasses entirely into aqueous solution under conditions of 2.5 MPa and170° C. The concentration of NH₃ in the Quenching Column is maintainedabove 13% by weight.

The aqueous solution output from the Quenching Column contains 7.8% byweight of melamine, less than 2500 ppm of OAT and less than 10 ppm ofpolycondensates. It is brought to almost atmospheric pressure and 45° C.in the Crystallizer. Under these conditions, 320 Kg/h of high-puritymelamine (99.95% by weight) crystallizes and is separated in theLiquids/Solids Separator and dried. In the Liquid/Solid Separator, 4.85m³/h of mother liquor are recovered, of which 4 m³/h return directly tothe Quenching Column to dissolve the melamine coming from thePost-reactor, while the remaining 0.85 m³/h are distilled in theDe-Ammoniating Column where 70 kg/h of anhydrous ammonia and, asside-stream, 90 Kg/h of aqueous ammonia solution containing the CO₂ arerecovered.

The de-ammoniated mother liquor from the De-Ammoniating Column,containing less than 700 ppm of NH₃, is cooled to 50° C. and the pHadjusted to the value of 7 by the addition of a small quantity of CO₂ inorder to reduce the solubility of OAT to a minimum and cause its almosttotal precipitation. The precipitated OAT is separated by filtering andeliminated from the water cycle. The filtrate, consisting in a 1%melamine solution containing less than 200 ppm of OAT, is recycled tothe Quenching Column, thus recovering the melamine contained in it.

While the invention has been described in connection with the abovedescribed embodiment, it is not intended to limit the scope of theinvention to the particular forms set forth, but on the contrary, it isintended to cover such alternatives, modifications, and equivalents asmay be included within the scope of the invention.

1. A process for the production of high-purity melamine comprising thesteps of: (a) pyrolyzing urea in a reactor operating at a temperaturecomprised between 360 and 420° C. and at a pressure higher than 7 MPa,the reactor producing reaction products that comprise a gaseous phaseand a liquid phase; (b) feeding the liquid phase from the reactor to adownstream post-reactor, the liquid phase from the reactor containingmelamine, non-reacted urea, intermediate oxidized products of pyrolysis,dissolved carbon dioxide, and products of de-ammoniating condensation ofmelamine (“polycondensates”), the post-reactor operating atsubstantially equal conditions of temperature and pressure as thereactor and producing a liquid product, gaseous superheated anhydrousammonia being fed under pressure to the post-reactor in order toeliminate the dissolved carbon dioxide and under conditions suitable forcompleting the pyrolysis reaction of the urea, for transforming themajority of the intermediate oxidized products of pyrolysis intomelamine, and for decreasing the polycondensates concentration; (c)dissolving the liquid product from the post-reactor in an aqueoussolution in a quenching column in the presence of ammonia and underconditions of temperature and residence time such to provide a solutionsubstantially free of polycondensates, the solution substantially freeof polycondensates being successively subjected to crystallization toprovide purified melamine in solid phase and an aqueous mother liquorthat contains melamine and reduced quantities of the intermediateoxidized products of pyrolysis, the mother liquor being recycled for thegreater part directly to the quenching column without any furthertreatment; and (d) subjecting the anhydrous gaseous phase coming fromthe reactor and post-reactor to washing with molten urea, therebyrecovering the melamine contained in the anhydrous gaseous phase asvapor, prior to returning the anhydrous gaseous phase to the ureasynthesis plant for the recovery of the ammonia and carbon dioxidecontained in it.
 2. The process of claim 1, wherein the mother liquornot directly recycled to the quenching column is subject to treatment ina de-ammoniating column for the recovery of the ammonia and theelimination of the carbon dioxide dissolved in it, and wherein themother liquor not directly recycled to the quenching column issuccessively sent to a precipitation and separation section forprecipitating and separating the intermediate oxidized products ofpyrolysis, the precipitation and separation section having the functionof maintaining the concentration of the intermediate oxidized productsof pyrolysis in the mother liquor constant at a level below saturation.3. The process of claim 1, wherein the gaseous, anhydrous superheatedammonia is blown into the post-reactor in a ratio ranging from 1:10 to1:1 with respect to the liquid phase from the reactor.
 4. The process ofclaim 1, wherein the residence time in the post-reactor of the liquidphase from the reactor is comprised between 0 and 2 hours.
 5. Theprocess of claim 1, wherein melamine is recovered at reactor pressureand in anhydrous conditions by washing the gaseous phase containingmelamine from the reactor and post-reactor with molten urea.
 6. Theprocess of claim 1, wherein the aqueous solution in the quenching columndissolving the liquid product from the post-reactor is maintained at atemperature of 160–170° C. in the presence of ammonia, the ammoniahaving a concentration above 10% by weight, the liquid product having aresidence time of less than 30 minutes.
 7. The process of claim 2,wherein the aqueous mother liquor subject to treatment in thede-ammoniating column and in the precipitation and separation section isin an amount comprised between 0 and 20% of the aqueous mother liquorrecovered after crystallization.